Increadable slowness on the UI

After a recent forced upgrade to OSS 5.0 from 4.5.6
there are many of our users experiencing problems with IE6 and zimbra. that they did not have before

Issues are more prominent in windows 2000 IE6 sp1 boxes, but even in our vista machines noticiable.

Basically, when inputting text into certain fields, ie will jump to 100% cpu usage, and text will take 10-15 seconds after being typed before populating the field.

The administration console is by far the worst, it is nearly impossible to add a new user and not have it take 3-5 minutes to do so. is there anyway to tell what is causing this slowness or how to fix it?